ABSTRACT Most cancer deaths are caused by distant metastasis. Yet the mechanisms that regulate distant metastasis are poorly understood. Metastasis is a very inefficient process in which few disseminated cancer cells survive, and even fewer proliferate, but it is not known why. We developed a patient-derived xenograft (PDX) assay in which melanomas engraft very efficiently and spontaneously metastasize. Using this assay, we discovered intrinsic differences in metastatic potential among melanomas from different patients. Some stage III melanomas are ?efficient metastasizers? that spontaneously form distant metastases in patients and in NSG mice while others are ?inefficient metastasizers? that do not form distant macrometastases in patients or in NSG mice under the same experimental conditions. Using this assay, we discovered that melanoma cells experience a spike in reactive oxygen species (ROS) during metastasis and that distant metastasis is limited by oxidative stress. Successfully metastasizing cells undergo reversible metabolic changes during metastasis that increase their capacity to withstand oxidative stress, including increased folate pathway dependence. However, the mechanisms that confer differences in metastatic potential upon melanomas from different patients have not yet been identified. We hypothesize that efficiently and inefficiently metastasizing melanomas have intrinsic metabolic differences that reduce oxidative stress in efficient metastasizers. One impediment to testing this hypothesis is that melanomas from patients grow poorly at clonal density in known culture conditions, preventing certain approaches for studying cancer metabolism and the use of CRISPR gene editing (because single cell-derived clones could not be screened or expanded). We spent years developing a culture medium in which single melanoma cells from patients form tumor organoids (PDOs). This capability raises the general question of whether metabolism and oxidative stress resistance are regulated similarly in PDXs and in PDOs. To address this question, we will compare, side-by-side, the biological properties and metabolic regulation of efficient and inefficient metastasizers in PDXs and PDOs. We will test if efficiently metastasizing melanomas have lower ROS levels or markers of oxidative stress, or increased use of the folate or pentose phosphate pathways, as compared to inefficient metastasizers, and whether this promotes metastasis in PDXs or migration/invasion in PDOs. We will also test if MCT1, a lactate transporter, promotes metastasis and whether efficient metastasizers reduce oxidative stress partly through lactate exchange. Finally, we will test if there are intrinsic differences in mitochondrial function between efficient and inefficient metastasizers that reduce ROS generation. By comparing the biological properties and metabolic regulation of each melanoma in PDX and PDO assays, we will assess the strengths and weaknesses of PDX and PDO assays for studying cancer metabolism and biological differences among patients. These results have the potential to identify new mechanisms that regulate metastasis, new aspects of cancer metabolism, and strategies to block progression.